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Yasuhiro KIMURA
111 lines
4.8 KiB
HTML
111 lines
4.8 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Locking with transactions: two-phase locking</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="lock.html" title="Chapter 17. The Locking Subsystem" />
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<link rel="prev" href="lock_notxn.html" title="Locking without transactions" />
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<link rel="next" href="lock_cam_conv.html" title="Berkeley DB Concurrent Data Store locking conventions" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Locking with transactions: two-phase
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locking</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="lock_notxn.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 17. The Locking Subsystem </th>
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<td width="20%" align="right"> <a accesskey="n" href="lock_cam_conv.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="lock_twopl"></a>Locking with transactions: two-phase
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locking</h2>
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</div>
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</div>
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</div>
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<p>
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Berkeley DB uses a locking protocol called
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<span class="emphasis"><em>two-phase locking (2PL)</em></span>. This is the
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traditional protocol used in conjunction with lock-based
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transaction systems.
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</p>
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<p>
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In a two-phase locking system, transactions are divided into
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two distinct phases. During the first phase, the transaction
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only acquires locks; during the second phase, the transaction
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only releases locks. More formally, once a transaction
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releases a lock, it may not acquire any additional locks.
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Practically, this translates into a system in which locks are
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acquired as they are needed throughout a transaction and
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retained until the transaction ends, either by committing or
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aborting. In Berkeley DB, locks are released during <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a>
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or <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a>. The only exception to this protocol occurs
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when we use lock-coupling to traverse a data structure. If the
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locks are held only for traversal purposes, it is safe to
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release locks before transactions commit or abort.
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</p>
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<p>
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For applications, the implications of 2PL are that
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long-running transactions will hold locks for a long time.
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When designing applications, lock contention should be
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considered. In order to reduce the probability of deadlock and
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achieve the best level of concurrency possible, the following
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guidelines are helpful.
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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When accessing multiple databases, design all
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transactions so that they access the files in the same
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order.
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</li>
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<li>
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If possible, access your most hotly contested
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resources last (so that their locks are held for the
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shortest time possible).
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</li>
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<li>
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If possible, use nested transactions to protect the
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parts of your transaction most likely to
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deadlock.
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</li>
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</ol>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="lock_notxn.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="lock.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="lock_cam_conv.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Locking without transactions </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Berkeley DB Concurrent Data Store locking conventions</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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